1. Field of the Invention
The present invention relates to backlighting systems for liquid crystal displays (LCDs), and more particularly to an integrated multi-capacitor divider network for use with a pulse-width-modulation (PWM) inverter controller for providing precise control of the voltage supplied to cold cathode fluorescent lamp (CCFL)-based backlighting systems.
2. Description of the Prior Art
LCDs are used in notebook computers, personal digital assistants (PDAs), automotive instrument panels, desktop displays, and other applications where size and performance are important. These LCDs are often illuminated in low ambient lighting conditions by a back- or edge-lighting system that uses one or more cold cathode fluorescent lamps (CCFLs) positioned at the edges of the display. Power for the CCFLs is normally provided by either a Buck/Royer oscillator inverter or a pulse-width-modulation (PWM) inverter controller that includes a capacitor charge pump circuit. Both the Buck/Royer oscillator and the PWM inverter controller convert a low-voltage direct current (DC) source into a high-voltage, high-frequency, quasi-sine wave that is required to ignite and power the CCFL. A typical igniting voltage is 1,500 VAC (RMS) and a typical operating voltage is 800 VAC (RMS), and the average CCFL tube life is 10,000 to 20,000 hours.
The Buck/Royer oscillator inverter uses a ballast capacitor in the range of 12 to 22 picofarads connected to the secondary of a high-voltage transformer and in series with the CCFL, in order to drop excess transformer voltage after the CCFL has ignited. Given a CCFL with 10 picofarads of nominal parasitic capacitance, the ballast capacitor and CCFL form a voltage divider such that only about 70 percent of the transformer output voltage is transferred through the ballast capacitor to ignite the CCFL. Therefore, the Buck/Royer oscillator inverter must develop 30 percent more voltage to ignite the CCFL, thus requiring a larger, more expensive transformer and related circuitry.
The PWM inverter controller uses a much larger DC bypass capacitor in the range of 100 nanofarads connected in series with the CCFL, so that virtually 100 percent of the transformer voltage is available for CCFL ignition. A capacitor charge pump circuit comprises a pair of capacitors connected to form a voltage divider, connected in parallel with the CCFL. Typically, one of the capacitors is a high-voltage (e.g., 3 kilovolt) NPO-type with a nominal capacitance of 5 to 33 picofarads. The other capacitor is a low-voltage (e.g., 50 volt) X7R-type with a nominal capacitance of 10 nanofarads, with the exact value determined by the desired ratio between the capacitors. The ratio of the values of these capacitors further determines the steady-state drive voltage across the CCFL, thereby determining the brightness of the illumination. Active open circuit voltage regulation is provided by non-dissipative voltage feedback.
The capacitors used in the charge pump circuit of the PWM inverter controller typically have a tolerance of five to 10 percent of their nominal value. Stated differently, if the nominal capacitance of the high-voltage capacitor is 10 picofarads, the actual value may range from nine picofarads to 11 picofarads. Because the steady-state drive voltage depends directly on the ratio of the capacitances, if one capacitor has an actual value on the high side of its tolerance, and the other an actual value on the low side of its tolerance, the resulting steady-state voltage and corresponding illumination can vary by up to 22 percent. This will cause variations and non-uniformity in the amount of light generated by the CCFL.
Accordingly, there is a need for an integrated multi-capacitor network having tolerances and temperature coefficients that vary in the same amount and direction, thereby assuring a predictable CCFL illumination.